1. Field of the Invention
This invention relates to a noise shaping circuit and, above all, to a noise shaping circuit advantageously employed in, for example, a 1-bit D/A converter.
2. Related Art
Recently, an oversampling type 1-bit D/A converting system is attracting attention as a high precision D/A converting system employed in audio equipment or the like. The basic construction of this type of the D/A converting system is shown in FIG. 1.
In this figure, digital signals supplied to an input terminal 101 are oversampled at a suitable oversampling factor by an oversampling circuit 102, constituted by a digital filter or the like, before being transmitted to a noise shaping circuit 103. In this noise shaping circuit 103, the digital signals are requantized to an order of several bits, 1 to 5 bits under the current state of the art. At this time, noises produced on requantization, or quantization errors, are fed back so as to be shifted to a higher frequency side to produce a noise spectrum distribution which is suppressed towards the lower frequency. Output data of several bits from the noise shaping circuit 103 are converted by a 1-bit D/A converter 104, constituted by a PWM circuit or the like, into 1-bit waveform data, which are outputted at an output terminal 105. Although a D/A converter for conversion into multi-bit waveform data may be used in place of the 1-bit D/A converter 104, it is necessary to overcome such problems as non-linear distortion on differentiation, glitch or the like.
For realizing a wide dynamic range with this system, a wide dynamic range is required of the noise shaping circuit 103. Among the factors governing the dynamic range of the noise shaping circuit 103 are an operating rate f.sub.NS, the number of degrees N and the number of bits M of the requantizer. Although the dynamic range is improved by raising the operating rate f.sub.NS, limitations are imposed on the operating rate f.sub.NS by an upper limit value of the operating rate of semiconductor devices. It is therefore contemplated to improve the S/N ratio by raising the number of degrees N.
FIG. 2 shows a noise shaping circuit of a customary N'th degree type or N-tuple integration type. Output signals from the oversampling circuit 102 shown in FIG. 1 are supplied to an input terminal 111 of the noise shaping circuit shown in FIG. 2, and output signals from an output terminal 112 are transmitted to the 1-bit D/A converter 104 shown in FIG. 1.
An output of a quantizer 113 of the noise shaping circuit of FIG. 2 is taken out via a 1-sample delay device 114 so as to be fed back to an input side of the quantizer 113. An integrator of a first degree 116.sub.1 is connected between an input terminal of the quantizer 113 and an adder 115.sub.1 which in effect is a subtractor subtracting the feedback signal from the input signal. The integrator 116.sub.1 is made up of an adder and a 1-sample delay element and adapted for delaying an output of the adder by one sample and feeding back the delayed data to the adder. The above is the basic construction of the noise shaping circuit of the first degree. With increase in the number of degrees, a number equal to the number of degrees of sets each consisting of an integrator and a negative feedback adder is provided in tandem in the direction of the input terminal, so that a noise shaping circuit of an N'th degree may be provided by providing an N number of sets each consisting of an integrator and an adder. FIG. 2 shows a typical construction of the N'th degree noise shaping circuit in which an N'th adder or subtractor 115.sub.N is connected to the input terminal 111 and an N'th integrator 116.sub.N is connected between the adder 115.sub.N and the next (N-1)st adder 115.sub.N-1. Output signals of the quantizer 113 delayed by the 1-sample delay device 114 are supplied to the adders 115.sub.N to 115.sub.1 and resulting 1-sample delay output signals are subtracted from input signals to the adders 115.sub.N to 115.sub.1.
It is noted that if, in the N'th degree noise shaping circuit shown in FIG. 2, an input to the input terminal 111 is X, an output from the output terminal 112 is Y and an quantization error produced at the quantizer 113 is .epsilon..sub.0, the output Y is given by EQU Y=X+(1-z.sup.-1).sup.N .epsilon..sub.O ( 1)
However, with the above construction, if the number of degrees is three or more, the integrator is overloaded to produce an unstable operation.
Although a multi-stage noise shaping circuit may be contemplated, inputs to respective circuit stages represent quantization errors of the preceding circuit stages, so that outputs of the respective circuit stages represent noise components. Since noise components of the second circuit stage et seq. are summed to the ultimate output, the dynamic range tends to be deteriorated.